1. Field of the Invention
The present invention relates to a method for the formation of a plug in a petroleum well extending from the earth""s surface or the sea floor to a petroleum reservoir, which well is lined with casing. The invention further relates to a tool for milling an opening in a casing of a petroleum well, and a plug for plugging casing of a petroleum well.
2. The Prior Art
Petroleum wells for the exploitation of gas or oil normally consist of an upper and outer conductor, which forms the base of the well, an upper casing located into and in extension of the conductor, and further down in the well more casings which are located into and overlaps the above casing. A production tubing string is located in the middle of the well for transporting petroleum from the bottom of the well to the earth""s surface or the sea floor. Annuli are formed between the different casings.
Some wells are test wells which are only used for a shorter period prior to the production from a reservoir, and thus will be plugged after testing. A successful well will normally be temporally plugged before the production starts, while a xe2x80x9cdryxe2x80x9d well, i.e., a well in which the hydrocarbon content is too small to be worth producing, will be plugged forever. Even the highest producing well will after some time be empty and abandoned, and thus all wells will sooner or later have to be plugged. For this purpose normally concrete plugs are used. In the following concrete plugs are intended to mean plugs constructed of a cement-based material.
Normally two barriers are required between the reservoir and the environment to ensure that there will be no blow-out or leakage of petroleum to the environment. When using concrete plugs, this means that one plug is located in the area of the reservoir. Preferably the second plug should also be located close to the reservoir, but this would mean that the concrete plug would have to be located in the casing, which is fairly smooth and does not provide much anchoring for the concrete plug. The second plug is therefore normally located on top of the well.
In case of permanently abandoned wells, governmental regulations in some areas require that the upper part of the well be removed to a certain depth. For an offshore well this means that the upper part of the well must be milled away to this depth, whereupon a concrete plug is placed in the well. The milling is time-consuming and requires the use of a drilling rig. For an offshore well which shall be abandoned, this means that an offshore platform must be used for several days for plugging a well. The plugging of offshore wells is thus very costly.
Concrete shrinks during curing, which means that cracks, pores and thin annuli between the concrete plug and surrounding walls of the well my be formed. Further, the long-time resistance of concrete to high pressure, high temperature and various chemical substances is uncertain, and thus the use of concrete plugs is linked to a future risk of leakage.
Another problem related to concrete plugs in offshore wells is that the sea floor in some areas sinks due to the exploitation of hydrocarbons. This subsidence causes motion in the ground, which causes stresses in the, concrete plugs, which again contributes to the cracking of the concrete. The subsidence of the sea floor also increases the pressure of the reservoir. Both effects contributes to the risk of leakage through the plugs, which of course is highly undesirable.
Experience supports the above considerations. In fact leakage is a big problem for a large number of wells which have been plugged with concrete plugs.
A further problem with plugging of wells is linked to the production tubing string, which is normally lifted out of the well prior to plugging. The production tubing will after some time get a radioactive scaling, and from an working environmental view it is thus desirable to let the production tubing string stay in the well.
GB 2 275 282 discloses a method for securing a suspended sub-sea well by setting a packer in the casing thereof, the packer having a perforating gun suspended therefrom. The gun is fired to perforate the casing, and then concrete is injected in the annulus behind the casing. The well is then sealed and the casing above the packer can be cut away. As concrete is used as a plug material, this plugging method does not solve the above problems related to concrete plugs.
The object of the invention is to provide a method for the formation of a plug in a petroleum well in which the above problems are reduced or eliminated. A particular object is to provide a method for the formation of a plug which can be carried out without the need for a drilling rig. A further object is to provide a tool and a plug which are favourable in the method. These objects are achieved by a method for the formation of a plug in a petroleum well, a tool for milling an opening in a casing and a plug for plugging casing as mentioned in the introductory part of the description, which method, tool and plug is characterised by the features of the claims. The term xe2x80x9cmillingxe2x80x9d is meant to include both xe2x80x9cmechanically or electrically removingxe2x80x9d and xe2x80x9chydraulically or electrically activatingxe2x80x9d.
Thus the invention relates to a method for the formation of a plug in a petroleum well extending from the earth""s surface or the sea floor to a petroleum reservoir, which well is lined with a casing. According to the invention, at least one opening is formed in the casing at a distance from the earth""s surface. Then a liquid curable resin is provided in at least a portion of the opening and an adjacent area of the well, whereby the resin after curing forms a plug in the well.
Preferably a production tubing string, which normally will be present in the well prior to the formation of the plug, is cut below the area of the plug, and the production tubing string above the cut is lifted out of the well. The production tubing string below the cut is left in the well.
The opening or openings in the casing may be formed by a well perforation tool, forming a plurality of small openings, a water jet tool or a mechanical machining tool. The tool may be supported by a drill pipe string or coil tubing, jointed pipe or wireline. In a preferred embodiment the tool is a milling tool which is suspended from a coil tubing, jointed pipe or wireline, and which is driven by a hydraulic motor which is energised by hydraulic fluid supplied through the coil tubing or the like.
Preferably the opening in the casing is formed in the entire circumference of the casing, and a mechanical packer is set in or right below the opening in the casing, whereupon the liquid resin is placed on top of the packer, the packer thereby forming a basis for the resin plug. In this way a resin plug in which the packer is integrated in the plug is formed. After the resin is cured, a concrete plug may be formed on top of the resin plug. Instead of a mechanical packer also an inflatable packer may be used.
As mentioned above, a well is normally plugged by two plugs. However only one plug or more than one plug may be used, depending on governmental regulations and practise. Typically the invention will be used for an upper plug, while a lower plug is made according to the prior art, but the invention can of course be used for any plug in a well plugged by any number of plugs.